1. Industrial Field of the Invention
This invention relates to a slag treatment material for modifying the properties of iron slag and steel slag, and, more particularly, to an improved slag treatment material which can effectively act to prevent, when a cooling process is applied to such iron slag or steel slag as blast furnace slag, the powdering of the slag due to disintegration of the generation of yellowish turbid water when slag lumps come into contact with water.
2. Prior Art
A steel slag, particularly stainless steel slag, which has a basicity (weight ratio CaO/SiO.sub.2) of substantially 1.5 or more, has a property whereby the 2CaO.SiO.sub.2, phase changes from an .alpha.-type phase to an .alpha.'-type phase, and then changes to a .gamma.-type phase or .beta.-type phase when the slag is subjected to a cooling process. In many cases, when the slag changes from the .alpha.'-type phase to the .gamma.-type phase, a volume expansion of substantially 14% results. As it well known, this causes the slag to powder into fine particles.
This powdering phenomenon worsens the working environment, and disturbs further utilization of slag. Present stainless steel manufacturers are therefore faced with serious problems in regard to the treatment of slag.
It has long been a problem for stainless steel manufacturers to find a method of preventing the powdering of slag and of solidifying it since the discharged slag can be effectively utilized as a secondary material in such applications as engineering aggregates for road construction and so on.
Known methods of preventing the powdering of slag can be exemplified as follows:
1 a method in which slag is made into water-granulated glass when the residue slag is discharged;
2 a method in which slag is modified to form a material which mainly comprises (CaO.SiO.sub.2 and has a basicity of 1.5 or less (in practice this can be slightly varied according to the composition of the slag);
3 a method in which the phase change from an .alpha.'-type phase to a .gamma.-type which results in a great change in density is controlled and the phase change from an .alpha.'-type phase to a .beta.-type phase is activated.
However, in regard to 1, at the time of water granulation, a phreatic explosion can occur due to the presence of molten metals carried at the time of discharge of the slag, and since water granulated glass is a soft material, it does not have sufficient strength to serve as an engineering aggregate. Therefore, this method 1 has not yet been put into practical use although it has been partially tested.
In regard to 2, although some additives designed to modify the properties of the SiO.sub.2 containing material have already been placed on the market, they require the installation of supplying facilities and stirring facilities since a large quantity of SiO.sub.2 needs to be employed equivalent to substantially 20% of the molten slag. Furthermore, the slag viscosity is increased according to the drop in temperature of the molten slag following the addition, and this is not suitable from the viewpoint of workability and total cost.
The method 3, that is, bringing about a phase change from an .alpha.'-type phase to a .beta.-type phase has been studied for many years and a variety of methods have been disclosed. One of these methods, which is the most effective and assured method available at present. [see Japanese Patent Laid-Open No.43690/1978 and the Kawatetsu Engineering Report Vol. 18, No. 1 (1986) 20 to 24] is one in which Si.sup.4+ ions are replaced by B.sub.3+ ions having a smaller diameter than that of the Si.sup.4+ ions contained in the slag.
However, the above-described conventional boron type of slag powdering preventing material is in the form of fine powder and a dehydration/vaporization reaction occurs at the time of contact with the molten slag since the boronic slag powdering preventing material is one containing water. As a result of this, a blowing phenomenon of the slag powdering preventing material is generated, causing the working environment to become excessively worsened and sometimes dangerous. Therefore operation of the work is very difficult.
Furthermore, since the conventional boron type of slag powdering preventing material differs significantly in the chemical composition and properties from slag, differences in viscosity and density from the fused slag can be easily generated, that is, the so-called affinity between the slag and the slag powdering preventing material is not sufficient and thereby the diffusing/mixing performance is insufficient. As a result of such disadvantages, the boron type of slag powdering preventing material cannot be put into practical use although there have been some proposals therefor.
A second problem arises in such iron or steel slag as blast furnace slag in that so-called "yellowish turbid water" is generated when the slag comes into contact with water such as rain or gutter water.
As is well known, blast furnace slag is widely used as various aggregates, particularly as road beds. However, it has been confirmed that if the percentage of sulfur contained in the slag is high, a mistake in the conditions or methods for use will cause yellowish turbid water and hydrogen sulfide smells to be generated due to rain water or gutter water which has been brought into contact with the slag. Particularly in order to ensure that the slag quality for road construction is free from such problems, it is a rule that such slag should not generate any yellowish turbid water or hydrogen sulfide smells. In order to evaluate this fact, a color identification test is employed, and slag should satisfy this test (JIS A 5015 made public on Nov. 1).
The phenomenon of generation of yellowish turbid water is caused from the elution of sulfur (S) contained in the form of calcium sulfide (CaS) which is contained, as a major part, in slag, and is due to generation of yellow polysulfide (such as CaSx) after being applied to a hydrolysis process. Known methods of preventing generation of yellowish turbid water can be exemplified as follows:
1 a method where slag is subjected to aging in which it is oxidized by water and air until it is stabilized;
2 a method in which an oxidant is added to molten slag;
3 a method in which slag is treated with CO.sub.2 so that the surface of the slag is stabilized; and
4 a method in which the cooling speed of slag is raised.
In regard to 1, aging treatment takes almost one to three months to be completed, requiring the use of a very broad space (slag yard) for storage.
In regard to 2, several methods can be exemplified such as a method in which a high degree of ferrous oxide is added or a method in which a gas containing oxygen, such as air, is added. However, this method is not preferable since poisonous SO.sub.2 gas is generated due to the reaction. Furthermore, with this method, the generation of the yellowish turbid water cannot be sufficiently prevented.
In regard to 3, although the surface of slag can be stabilized, when it is used as ballast this method is disadvantageous in that fused sulfides can again overflow from the fractured surface when it is crushed.
The method 4 is a method in which glass is prepared by degrading, diffusing and rapidly cooling down fused slag so that the included sulfur component is prevented from oozing out. However, this involves such problems as a deterioration in strength and necessitates granulation to below a specific viscosity to form the glass, allowing it to be used as a material for thin aggregates, but making it very difficult to be used as rough aggregates.
Although other methods have been disclosed in which iron, manganese or zinc is, as an effective component, added so that sulfides are fixed, these methods have not been put into practical use due to high costs and insufficient effects.
Therefore, at present, the method 1 in which slag is subjected to aging is the only available method for preventing generation of yellowish turbid water from slag.